Workpiece temperature equalization in rolling of steel

ABSTRACT

THIS PATENT DISCLOSES PROVIDING A HOT-STRIP ROLLING MILL, AT THE HOLDING TABLE PRECEDING THE FINISHING TRAIN, WHICH TEMPERATURE-SENSING MEANS AND FAST-ACTING ELECTRIC-INDUCTION HEATING MEANS RESPONSIVE THERETO FOR ELIMINATING COLD SPOTS IN THE WORKPIECE TO PERMIT IT TO BE PROCESSED THROUGH THE FINDING TRAIN.

United States Patent processed through the [72] inventors Francis H- Bricmonr, 905 Bridgewnter 2,177,971 10/1939 Worthington Drive, Pittsburgh, Pa. 15216: Ervin R. 3,035,143 5/1962 beatherman v mu nmp mm, 3,080,468 3/1963 Wuczkowski Pittsburgh, Pa. 15220 3,344,648 10/1967 Gray [21] Appl. No. 760,225 3,411,332 11/1968 Cook [22] Filed Sept. 17,1968 3,444,346 5/1969 Russell et a1 [45] Pltented June 28, 1971 3,471,673 10/1969 Frostick Primary Examiner-Milton S. Mehr s41 WORKPIECE TEMPERATURE EQUALIZATION 1N PM and ROLLING OF STEEL 3 CHI, 3 Dru-h Ifis.

[52] 0.8.61. 72/13, 72/202, 219/1061, 219110.77 [51] int. Cl. l2": 37/10 [50] held I 72/13, 202, ABSTRACT; This went digcloggg providing a hot-strip I061. -6 10-77 rolling mill, at the holding table preceding the finishing train,

with temperature-sensing means and fast-acting electric-in- [56] m duction heating means responsive thereto for eliminating cold UNITED STATES PATENTS u in the workpieceto permit 1mm 1,718,806 I 6/1929 Wining 72/13 finishing train.

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WORKPIECE TEMPERATURE EQUALIZATION IN ROLLING OF STEEL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to apparatus for heating steel workpieces, and in particular, to apparatus for. reheating slabs, bars, or billets during a size-reduction process.

2. Description of the Prior Art It is a common practice to heat steel slabs in a continuous pusher-type furnace wherein, as the steel slabs travel through the furnace, they are supported upon water-cooled skids. Such pusher-type continuous slab-heating furnaces are customarily used in connection with hot-rolling equipment, such as a continuous hot-strip mill serving to reduce steel slabs to strip of hot-band gauge, such as about 0.1 inch thick.

It is known that the water-cooled skids cause the development of cold spots in the vicinities of the portions of the steel slab with which they come into contact. These cold spots tend to cause difficulties in the subsequent rolling of the steel. They are somewhat harder than the surrounding metal and thus tend not to be reduced to the same extent'as the surrounding metal when passing through a given rolling pass.

In the past, various measures have been adapted to lessen the problem, such as using a heating furnace with staggered skids and/or providing a soaking zone in the continuous heating furnace. Although such measures are helpful, they do not completely overcome the cold spot" problem.

It sometimes happens that during the subsequent rolling process, and in particular by the time that a piece of steel rolled from a slab is about to enter the finishing train of a hotstrip mill, the temperature of the piece becomes so low that further rollingis not feasible. Prior to this invention, it has been necessary to scrap the piece whenever this occurs.

Difficulty with cold sports or skid marks occurs most frequently with older hot-strip mills that are being operated at about their intended capacity, or a little higher. When high production is not required, it is naturally possible to retain a slab within the soaking zone of the heating furnace for a period of time sufficient to alleviate or overcome the skid mark" problem. With modern furnaces provided with sophisticated fast-acting guage-control systems, there is again less need for the invention. With an older existing mill that is expected to yield high production, there is often need for the present invention, since the various alternatives are all less advantageous. To provide an entire new mill is entirely too expensive. To provide the existing mill with an additional new furnace is quite costly, and it involves the expenses of upkeep of the new furnace, which are not small. Installing modern guage-control equipment is also possible, and not so expensive in equipment cost as a new furnace, but the mill must be taken out of production for a considerable period of time if this solution to the problem is adopted. Less costly than any of these is the solution proposed in accordance with the present invention, and the shutdown that is required for its implementation is relatively short.

It is usual for a steel workpiece heated in a furnace having water-cooled skids running longitudinally thereof to be discharged to a rolling line wherein the workpiece proceeds in a direction crosswise to that of its passage through the furnace with water-cooled skids. This means that the rolling-mill stands encounter, in rolling a piece so heated, first a warmer portion, then a cooler portion corresponding to a first one of the skids, then a second warmer portion, and then a cooler portion corresponding to a second one of the skids, and so on. As the workpiece is reduced in thickness, the length of strip corresponding to each of the skid marks is increased. Although relatively little difiiculty occurs in the roughing stands, where the skid-mark regions of the strip are relatively small, considerable difficulty may be encountered in the finishing stands. When these are provided with modern guagecontrol equipment, such equipment is sometimes hard pressed to adjust the. screwdownsettings quickly enough. to keep the product gauge within allowable tolerances. A mill without such gauge control simply produces strip that increases in thickness in the cold-spot areas-until the production rate slows down to the point where the time of retention in the soaking area of the furnace is high enough to eliminate the cold spots. Such low production rates are, of course, undesirable for other reasons.

It is known that steel workpieces can be heated by means of an induction coil positioned therearound and provided with alternating electrical current, and that the heating that can be so obtained is quite repaid and effective, so that it is necessary in many instances to take care to limit suitably the duration of the heating.

BRIEF SUMMARY OF THE INVENTION In accordance with the invention, the problems of cold spots on slabs and providing for the convenient reheating of breakdowns that have become too cold to be finished in the usual way in a steel rolling operation, are overcome by providing, between the exit of a slab-heating furnace and the beginning of a finishing train in a hot-strip rolling mill, and preferably in the vicinity of the latter, means for sensing the temperature of the steel workpiece and induction-coil heating means responsive to the temperature-sensing means for raising the temperature of the steel workpiece and/or eliminating cold spots therein.

BRIEF DESCRIPTION OF THE DRAWINGS A complete understanding of the invention may be had from the foregoing and following description thereof, taken together with the appended drawings, in which:

FIG. 1A represents schematically one portion of a hot-strip rolling mill provided with the improvement of the present invention;

FIG. 18 represents schematically a further portion of a hotstrip rolling mill provided with the improvement of the present invention; and

FIG. 2 is a schematic representation of a portion of a hotstrip rolling mill, showing an alternative embodiment of the in vention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1A and 18, there is shown schematically a hot-strip rolling mill, such as is used for rolling slabs of steel to hot-band gauge, and in particular, there is shown a slab yard 2, wherein there are stored cold slabs 4. The slabs 4 are lifted by means of a slab yard crane 6 and conveyed to the charging table 8 of one or more furnaces 10. The furnaces 10 are of the kind that continuously heat a plurality of slabs 4, with in- I dividual slabs being pushed through the furnace by means of pushers 12. The slabs issue from the furnaces I0 and abut against stops [4, whereafter they are conveyed through a processing line 16 comprising a first scale breaker 18, a highpressure water spray 20, and roughing stands 32, 34 and 36, provided with vertical edging rolls 38, 40 and 42, respectively. Having been processed by this equipment, a piece of steel emerges onto a holding table 46.

Subsequently, the steel is passed through a shear 48, a second scale breaker 50, a second high-pressure water spray 52, and a finishing train 54 comprising six tandem mill stands 56, passing therefrom under coolingwater sprays 58 and being then engaged by one of the coilers 60.

The equipment described above is conventional. In particular, the furnaces 10 are of the kind containing water-cooled skids running longitudinally thereof. As mentioned above, these water-cooled skids cause cold spots of the slab. The difficulties caused by these cold spots are not especially apparent during the roughing of the slab into a partially rolled slab, such as is obtained when the steel has reached the holding table 46, but the deficiencies of pieces of steel inadequately soaked in the furnace l0 become especially apparent during thepassage of the'steel through the finishing train 54.

It also sometimes happens. that by the time the steel has reached the holding table 46, it is too cool to be processed through the finishing train 54. It has hitherto been necessary to scrap a partially rolled steel slab that, by the time that it reaches the table 46, is too cool. This happens, most frequently, as the result of acobble'in the finishing train.

In accordance with the present invention, the above-indicated difficulties are overcome by providing, preferably in the vicinity of the holding table 46, but if desired, at another location along the line 16 between the exits of the furnaces and the' beginning of the finishing train 54, certain equipment comprising means for sensing the temperature of the steel workpiece W and means responsive to the temperaturesensing means for heating the workpiece W by means of electrical induction, so that the workpiece W may be speedily restored to a temperature at which it may be processed through the finishing train 54 or, in the event that the workpiece W has cold spots in it, these may be eliminated, in order that the processing through the finishing train 54 may be conducted a great deal more smoothly, i.e., with fewer variations in power requirements of the kind that are caused by temperature nonuniformity. A hot-strip rolling mill provided with means as indicated above has the further advantage that it is capable of producing higher-quality product, since a closer control of processing temperatures becomes possible, and this eliminates a substantial proportion of the variation in gauge that is encountered when the rolling of steel is practiced in accordance with methods known before the present invention.

The invention may be practiced, as indicated in FIGS. 1A and 18, by providing a contact thennocouple or radiation pyrometer 62 in the vicinity of the holding table 46, which.

serves as the above-mentioned temperature-sensing means. Of course, it will also be possible to use other equivalent temperature-sensing means.

lt is preferred that the temperature-sensing means be one of the kind that is capable of producing an electrical or pneumatic signal indicative of the temperature sensed. Such signal is then fed into a control circuit 64, serving to connect and disconnect from a suitable source of alternating electrical current 66 a coil 68 comprising the salient operating part of an induction-coil heating means. 7

The design of a suitable control circuit 64, as well as a selection of proper operating parameters for the induction-coil heating means (frequency and wattage of power supplied, number of turns in coil, dimensions of coil, etc.), are well within the skill of theart.

What has not been appreciated by those of ordinary skill in the art, prior to the present invention, is the substantial improvement in the operation of a hot-strip finishing mill that can be obtained, and at lower expense in comparison with providing and maintaining a modern, sophisticated guage-control mechanism, by providing, in the part of the line 16 between the exit of the furnace l0 and the beginning of the finishing train 54, the means 62 and 68, as shown and described above. Particularly, it has not been appreciated that, despite the homogenization of temperature nonuniformities that is done in the soaking portion of the furnaces l0 and despite the further homogenization of temperature that naturally occurs during the passage through the various line equipment between the first scale breaker l8 and the last roughing stand 36, there still exist, in slabs heated in a conventional continuous-type pusher-operated slab-heating furnace having the usual water-cooled skids, a sufficient number of temperature nonuniformities that, when they are eliminated as by means of the present invention, there is obtained a substantial improvement in the operation of the finishing train 54 and a substantial improvement in the quality of the strip produced by the hot-strip rolling process.

As those skilled in the art will wellappreciate, it is not essential that there be used a single temperature-sensing means (many may be used, each responsive to a particular portion of slab monitored thereby) or a single induction heating means. Either of these items may, of course, be duplicated to any desired extent, in order to suit requirements.

lt wrll, of course, be possible, within the scope of the invention, to use more than one sensing device, together with an adequately augmented control circuit, or if desired, more than one induction heating means. Those skilled in the art will appreciate that, within the scope of the present invention, the desired results may quite naturally be obtained in many different ways, particularly advantageous and unobvious ones of which are presented and defined in this application for patent. Moreover, though the present invention has been particularly described with reference to the rolling of steel slabs into strip of hot-band gauge, it will be clear to those skilled in the art that the invention may also be practiced in the rolling of bars or other steel workpieces. v

Reference is now directed to FIG. 2, which shows an alternative embodiment wherein the workpiece is electrical-re sistance heated. ltems bearing reference numerals the same as those used in FIG. 1 are,'of course, substantially similar to those used in the embodiment already described above. The control circuit 64 is operatively connected, as at 70, to a switch 72 in lines 74 leading from a suitable power source (not shown) to contact means 76 78 for feeding electrical current to the workpiece W if, as detected by the means 62, this is necessary. With resistance heating, the electrical current may be direct current, with (as indicated) the means 76 providing the positive potential and the means" providing the negative potential.

While we have shown and described herein certain embodiments of our invention, we intend to cover as well any change or modification therein which may be made without departing from its spirit and scope.

We claim: 1. A method for the size reduction of a metallic slab, comprising:

heating said slab while it is positioned on water-cooled skids within a furnace such that when the slab emerges from the furnace those portions of the slab which were in contact with the skids will be cooler than the remainder of the slab, I

at least partially reducing the cross-sectional area of said slab in a roughing mill,

measuring the temperature of said slab after it has passed through the roughing mill to determine if its temperature has fallen to the point where said cooler portions of the slab will cause variations in gauge above a predetermined acceptable level when the slab is subsequently reduced in a hot-strip finishing train,

electrically heating said slab if its temperature is at or below said point to raise the temperature of the slab to a level where said variations in gauge after finishing are acceptable, and

passing the thus-heated slab through a hot mill finishing train.

2. The method of claim I wherein said slab is electrically heated by passing saidslab through an induction heating coil and energizing said coil. v

3. The method of claim I wherein said slab is electrically resistance-heated. 

